1. Field of the Invention
The present invention relates to an apparatus for producing a glass preform and more particularly, to an apparatus for producing a glass preform by VAD (vapor phase axial deposition).
2. Description of Background Information
VAD is known as a method of producing a glass preform. The glass preform is produced by VAD as follows. A starting rod or a core rod is rotated by fixing an upper portion of the starting rod or the core rod to a shaft of a motor. Soot, which is caused by reactions of minute glass particles by an oxy-hydrogen burner containing gaseous raw materials, is grown axially on the rotating starting rod or the core rod and thus, porous glass acting as a core and a cladding is formed. The thus obtained porous glass is heated by a ring heater formed by high-purity carbon such that air bubbles contained in the glass particles, are forced out of the glass particles. As a result, a transparent optical fiber preform is obtained.
VAD includes a core forming step, in which porous glass acting as a core and a cladding is formed on a starting rod by gripping an upper portion of the starting rod. VAD also includes a jacketing step, in which, by gripping an upper portion of a core rod obtained by vitrifying the porous glass, soot is formed on the core rod sequentially downwardly so as to increase the diameter of the cladding. The jacketing step is performed for the purpose of setting a ratio of diameter of the core to that of the cladding to a proper value and is repeated a plurality of times in some cases.
In both the core forming step and the jacketing step, since soot is formed by gripping only the upper portion of the starting rod or the core rod, a lower portion of the starting rod or the core rod acts as a free end and thus, free vibrations of the starting rod or the core rod are likely to take place. As a result, uniform adherence of soot becomes difficult and thus, quality of the glass preform deteriorates.
As a countermeasure against vibrations of a rotor in a rotational machine, it has been a common practice that the number of rotations of the rotor and natural (e.g., resonant) frequency of the rotor are caused to deviate from each other. However, in sooting by VAD, since natural frequency of soot material obtained by depositing soot on the starting rod or the core rod varies with deposition of the soot, it is difficult to adopt this known countermeasure.
Therefore, in a sooting process of known VAD, no special consideration is given to the natural frequency of the soot material. Thus, even if vibrations of the soot material happen, the vibrations are allowed to continue until the soot material spontaneously deviates from a resonant state with the progress of deposition of soot.
Thus, in known VAD, variations of natural frequency of the soot material are not considered. In order to minimize causes of vibrations of the soot material, procedures that improve production accuracy have been pursued. For example, the starting rod or the core rod is performed prior to sooting so as to lessen the eccentricity of the soot material. However, these known procedures are quite troublesome and improvement of production accuracy is limited. Meanwhile, in response to a recent increase of size of the glass preform, variations of natural frequency of the soot material become greater, thereby resulting in an increased risk of resonance of the soot material.